We have cloned a segment of DNA that contains SV4O integrated into the genomic DNA of cultured human epithelial cell. The keratinocyte cell line studied was infected by the SV4O virus and has maintained the viral sequences in an integrated state over a period of several years. The oncogenic properties of the virus stem directly from the expression of the viral early genes which is responsible for the induction of transformation in the epithelial cell line. Once the transformation has occurred the cells exhibit unlimited growth in cell culture and can become "immortalized". We have cloned the integrated viral sequences from a genomic library created in a lambda phage vector from XbaI digestion of the genomic DNA from the transformed cells. We have derived sequence data from the 6.93 kb segment of DNA that has been cloned into the single- stranded sequencing vector mpl9 and we have characterized the host sequences flanking the viral DNA and the site of integration by homology to known genetic elements catalogued on the GENBANK and EMBL databases. Comparative analysis of virus integration in different cell lines should give us information on the integration site and the potential for altered regulation of genes adjacent to the integration site.